1. Field of the Invention
The present invention relates to an ink jet printing apparatus and a method of controlling preliminary discharge of the apparatus.
2. Description of the Related Art
An ink jet printing head discharges an ink droplet within a nozzle from a minute hole (hereinafter referred to as a discharge port) of the nozzle toward a printing medium to perform printing. As the printing head discharges the ink, the ink within the nozzle is decreased and a new ink is charged from an ink chamber by capillary force.
However, when the ink is not discharged over a long period of time, evaporation of moisture and a solvent of the ink may cause a color material to be precipitated at the discharge port and the precipitated color material may form a film at the discharge port, thus preventing a normal ink discharge. A kinetic energy produced to discharge the ink droplet by a printing operation of the printing head is consumed in breaking the film. As a result, the ink droplet cannot achieve a sufficient discharge speed and thus cannot be printed in a desired position on the printing medium.
To cope with this, the ink jet printing apparatus performs a preliminary discharge operation to move the printing head outside the printing medium and to discharge the ink at a predetermined location before a complete film is formed at the discharge port. By performing the preliminary discharge operation, the nozzle can be maintained in a condition to do a normal ink discharge during printing.
In general, a serial-scanning ink jet printing apparatus judges whether or not a predetermined time has passed since a previous preliminary discharge at the time of an inversion during reciprocating scanning of a carriage with a printing head, and performs such control as to move the printing head outside a printing medium for a preliminary discharge if the predetermined time has passed.
However, frequent preliminary discharge operations cause printing speed to be decreased, and also involve the disadvantage that consumption of the ink except in actual printing leads to a rise in running costs.
In order to eliminate such disadvantage, for example, Japanese Patent Application Laid-open No. 63-252748 discloses a control method, wherein the number of actuations of a plurality of nozzles is measured in a predetermined time, and if the value measured is less than a predetermined number, a preliminary discharge is performed, while if the value measured is equal to or greater than the predetermined number, the preliminary discharge is not performed. This method eliminates the preliminary discharge operation of nozzles undergoing frequent discharge operations, and therefore the time and amount of ink wasted in the preliminary discharge are reduced.
Also, Japanese Patent Application Laid-open No. 2004-082629 discloses a method, wherein the number of actuations of a plurality of nozzles is measured in a predetermined time, a judgment is made whether the value measured is equal to or greater than a predetermined number, and based on the judgment, a subsequent printable time is adjusted. And with each completion of a printing scanning, the adjusted printable time is compared with the time required for subsequent printing scanning, and a preliminary discharge operation is performed if the printable time is shorter. A control method like this does not perform the preliminary discharge carelessly with each printing scanning even when the time required for one printing scanning is equivalent to discharge-guarantee time of non-discharge nozzles. This enables the preliminary discharge to be efficiently performed without reducing throughput as much as possible.
The preliminary discharge of the ink jet printing apparatus is an important operation required to maintain image quality. On the other hand, however, the preliminary discharge can trigger an increased ink consumption and decreased throughput. Based on this standpoint, one of the problems in recent ink jet printing apparatuses is how efficiently and unwastedly the preliminary discharge operation is performed.
In recent years, there has been an increasing demand for so-called “margin-less printing” in which an image is formed on a printing medium without setting in any margin. And a number of arrangements or printing methods which enable such printing have already been proposed (refer to Japanese Patent Application Laid-open No. 2003-127353 and No. 2004-1416). When, in the “margin-less printing”, printing is performed in the vicinity of an endmost portion of the printing medium, support of the printing medium becomes unstable, which may cause conveyance accuracy of the printing medium to be decreased. Also, there are concerns that ink discharged running off the printing medium may contaminate an inside of the apparatus or of a non-printed area of the printing medium. Therefore, as disclosed in the above-mentioned publication, No. 2004-1416, for printing in the vicinity of the endmost portion of the printing medium, a method is generally adopted in which the number of nozzles to be practically applied to the printing is limited.
Even when such “margin-less printing” is performed, the preliminary discharge method described in either of the above-mentioned publications, No. 63-252748 or No. 2004-082629, has been conventionally adopted. However, with a limited number of nozzles for printing on the endmost portion, non-discharge nozzles which are not involved in printing always exist, and therefore the conventional preliminary discharge control methods are not able to bring about an efficient preliminary discharge operation. That is, during printing in the vicinity of the endmost portion of the printing medium, due to the nozzles which are not involved in the printing, more preliminary discharge operations than required are performed, and accordingly the ink is improperly consumed, resulting in a decreased throughput.